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Setting Planet positions on the Orbit™ Orrery The positions of the planets relative to each other and to the Sun are constantly changing. To set up an orrery, Heliocentric (Sun-centred) Longitudes are most useful. Monthly positions are given below. To position your planets cut out the degree scale printed oppostie and place it on your orrery. Then align the planets with the Longitudes given. N.B. The direction of zero degrees is not important for the activities in this leaflet. It is important if one wants to consider the positions of the stars relative to the planets or the height of the planets above the horizon due to the Earth’s tilt at different times of the year. More detailed models such as the Helios Planetarium cover these topics. 340 350 0 10 20 330 30 320 40 310 50 300 60 290 70 280 80 Sun 270 90 260 100 250 110 240 120 230 130 220 140 210 200 190 180 170 160 150 Heliocentric longitudes for setting up the Orbit™ orrery Jan 1st Feb 1st Mar 1st Apr 1st May 1st Jun 1st Jul 1st Aug 1st Sep 1st Oct 1st Nov 1st Dec 1st 2007 Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto 269 318 100 245 243 140 344 319 267 25 7 131 261 245 141 344 319 267 179 52 160 277 247 142 345 320 267 275 101 191 296 250 143 345 320 267 30 150 220 315 252 144 345 320 267 194 200 250 334 255 146 346 320 268 283 249 279 353 257 147 346 320 268 54 298 309 12 259 148 346 321 268 208 347 338 31 262 149 347 321 268 295 35 8 48 264 150 347 321 268 79 84 38 65 267 151 347 321 268 218 133 68 81 269 152 348 321 269 2008 Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto 308 183 100 96 272 153 348 321 269 104 233 131 111 274 154 348 322 269 227 279 161 124 277 155 349 322 269 319 329 191 138 279 156 349 322 269 116 16 221 151 282 157 349 322 270 238 66 251 164 284 159 350 322 270 330 138 250 347 158 258 114 164 214 262 311 359 280 309 339 8 39 69 178 191 206 220 236 251 287 289 292 295 297 300 160 161 162 163 164 165 350 350 351 351 351 352 323 323 323 323 323 324 270 270 270 270 271 271 © Cochranes of Oxford Ltd, 1999-2004. N.B. If the date is in the middle of a month, position the planets between the two numbers given. Page 1 © Cochranes of Oxford Ltd, 1999-2004. All rights reserved. Which planets can I see when? The diagram below shows why you can see different planets at different times. The diagram shows the positions of a person on the equator at evening sunset, midnight and morning sunrise as viewed from north of the ecliptic. The straight lines represent where the observer’s horizon cuts through the ecliptic (the plane of the planets’ orbits) to the East and West. The shaded area is the visible sky. Sun Sun Sunset Midnight Sun Sunrise The two inner planets, Mercury and Venus, can only be seen close to Sunrise and Sunset and not at Midnight. When planets are only visible in the morning or evening they are called “morning stars” or “evening stars” respectively. The outer planets (those farther from the Sun than Earth) may be seen at different times depending on their position. If used with the orrery set up with today’s longitudes, this method gives an approximate guide to which planets one could see tonight. This method is only approximate as the distances between the different planets and the Sun are not to scale. Looking at the Orbit Orrery lit up, two other factors that determine how easily a planet is seen are visible: - The larger planets reflect more light - The planets closer to the sun reflect more light. With the naked eye one can see the planets Mercury, Venus, Mars, Jupiter and Saturn. Other factors affecting visibility include the proximity of the planet to Earth at the time, cloud cover, physical obstructions and light pollution. Page 2 © Cochranes of Oxford Ltd, 1999-2004. All rights reserved.